Autoinjectors are designed to facilitate injection procedures over those required by manual use of common syringes and to secure a proper injection result highly independent of operational circumstances. Autoinjectors are typically used in non-hospital environments, sometimes in emergency situations, and by non-professionals like unskilled assistants or the patients themselves, which operator groups may include sick, disabled, elderly and child persons. The autoinjectors provide at least an automatic injection step in which stored energy, for example from a compressed spring, is released by a trigger to act on a syringe piston or plunger for expulsion of syringe content. Frequently the autoinjectors also provide an automatic penetration step in which stored energy is used for propulsion of the syringe from a rear position, in which the needle is hidden, to a front position, in which the needle is at least partially exposed, to thereby relieve the patient from the, sometimes fearful, task of inserting the needle through the skin and to secure an always appropriate penetration depth once the autoinjector front has been placed against the skin. Autopenetration and autoinjection may take place concurrently, e.g. in simple devices or for the intentional purpose of allowing for an over depth distributed injection. Normally it is desirable to limit injection until the needle has reached or is close to its target location. Still some known injectors try to obtain this feat with a single force system acting on syringe piston or plunger for both purposes, relying for sequencing on the normally lower needle penetration resistance than fluid ejection flow resistance. Yet impact, propulsion inertia and friction cannot prevent at least some leakage during penetration but above all, in case the penetration movement is prevented or jams, injection will entirely fail with preparation expelled on the skin or at improper depth. Hence more advanced injectors applies penetration force directly or indirectly on the syringe barrel, with single or dual drive systems, which requires some control mechanism disabling injection force application during most of the penetration phase and enabling injection force only after proper penetration. Autoinjectors may also provide an automatic needle retraction step in which stored energy, typically stored during the penetration movement in a weaker return counter-spring, acts to push the syringe back into the autoinjector after completed injection in order to relieve the user from the task and risk of withdrawal, to verify sequence completion to the user and to prevent inadvertent needle pricks after use. Again, this function may need a control mechanism enabling action of the return spring only after completed injection, normally accomplished by separation of the penetration and injection forces from the syringe at a certain forward extreme for the piston or plunger, freeing the return spring for action.
Most known autoinjectors are designed for use with a single syringe type or even a single specialized and adapted syringe type container in order to meet the various tolerance in dimensions, sizes and forces involved and these requirements become more pronounced when more of the advanced features outlined above are included in the injector. Yet there is a need for autoinjectors able to operate with a variety of syringe sizes, filling degrees, preparation viscosities, aging properties, temperature conditions, needles and flow characteristics. A manufacturer of a broad range of preparations may need a device useful for many container types and doses. Low cost preparations in particular cannot support development of a unique device or syringe container of its own and furthermore may require use of cheap standardized syringe types on the market with a selected minimum size for each dose. Patients on prescription of several pharmaceuticals may benefit from replacement of several devices for a single universal one. Manufacturers of injectors may find a broader market for their autoinjectors if compatible with container variations.
The above objects meet with numerous problems. Variations in size first require a syringe seat or carrier, not only able to accommodate and guide the various container movements with small lateral deviations, but also to secure appropriate start and end positions with respect to both the injector front and the injector mechanism. Variations in length or filling degree means differences in start positions for penetration and injection, either requiring a complicated device with adaptable start positions or long worst case dead runs for the mechanism, creating strong and potentially destructive impact forces or painful injection rates. The force requirements are highly variable. Variations in diameter, for example, means variations in injection force due to differences in piston cross-section surface, even at similar hydraulic flow pressures, as well as differences in piston to wall friction. Further broadening in force requirements is caused by differences in flow characteristics, such as resistance and obstructions in syringe opening, needle lengths or diameters as well as receiving tissue, and by differences in piston to wall friction, even at constant diameter, due to manufacturing tolerances and aging, typically resulting in increased friction due to an ongoing depletion of lubricant in the piston to wall contact surface. It is also well known that the first piston displacement requires a much higher "break-loose" force than continued motion. An again increased force may be desirable at the piston bottoming out to fully squeeze out container content, of special value at precise dosing or for expensive preparations. If the autoinjector drive systems are proportioned for the highest force required by all the abovesaid factors combined, they tend to be excessively strong for less demanding combinations, besides becoming generally bulky and ungainly. Applied in the penetration step the forces may damage or destroy smaller or weaker glass containers and counteract a safe penetration due to vibrations, shaking, recoil and rebound effects. Applied to the injection step extreme pressures may damage the container itself, deform the piston or blow away front sealing or attachments and most probably cause pain and bruises in the receiving tissue. As indicated above all these problems are exaggerated if the high forces are combined with inertia effects from substantial mechanism dead run. If, on the other hand, the autoinjector is provided with means for adjusting the force to the requirements of each container type, these problems can be reduced but instead a more complicated device results and an additional tuning step is expected of the user, defeating the basic simplicity, safety and rapidity reasons for using autoinjectors. Finally, a variety of container types, sizes and tolerances place severe demands on the control mechanism for sequencing the autoinjector phases, as the containers may require different locations and conditions for shift between enabled and disabled states.
Existing prior art does not seem to give much guidance to the resolution of the abovesaid problems. Infusion pumps, typically for slow speed administration in hospital settings, explicitly usable for syringes of different sizes are known, as exemplified by U.S. Pat. No. 4,652,260, U.S. Pat. No. 4,838,857, U.S. Pat. No. 4,976,696, U.S. Pat. No. 5,034,004 and U.S. Pat. No. 5,545,140, all relating to injection by electric motor means where speed can easily be controlled. Similar infusion pumps using mechanical drive means under hydraulic speed control are known from U.S. Pat. No. 3,474,787, U.S. Pat. No. 3,605,745, U.S. Pat. No. 4,430,079, U.S. Pat. No. 4,437,859, WO 88/10129 and GB 1,026,593 or under mechanical speed control from U.S. Pat. No. 3,279,653 and U.S. Pat. No. 3,886,938, although these references do not suggest any adaptations for syringes of varying size. Common to all infusion pump systems is that no penetration step is involved, and still less an autopenetration step, or any needle return step. Accordingly they provide no solutions in this regard or in connection with sequencing such steps. Nor are any solutions to be found in respect of the abovesaid force problems, due to the slow speeds and flow pressures involved and due to the common practice of initiating the infusion procedure by manually or automatically placing driver heads cautiously against the syringe plunger. Any overpressure arises so slowly that the infusion procedure easily can be halted, automatically or manually after delivery of an alarm signal.
Some autoinjector proposals try to cope with excessive forces by including mechanical dampers in the form of impact retarding springs, elastic gaskets etc., as exemplified by WO 94/13342, WO 95/04562 and DE 3,914,818. These proposals are not made for the purpose of allowing syringe variations and are entirely unsatisfactory and insufficient for the dramatically broadened force requirements in these connections. Also the other problems described are left unsolved,
The WO 95/31235 reference discloses an autoinjector which can be used with syringe subassemblies of different sizes and of standard type, having plunger shafts. However, no solution of general nature is given. For each type an adapted medical module has to be provided and the problem of allowing for different sized syringes in a common carrier is not addressed and expulsion of different doses require special stop surface arrangements in the modules. Further, no solution is given for such arrangements in connection with autopenetration force applied to the syringe barrel rather than its shaft or for automatic needle return. The arrangements described are unable to handle problems with high and varying injection forces.
Accordingly there remains a need for autoinjector designs better adapted for use with great variations in syringe size and type and with improved capabilities for handling the problems outlined.